In a wireless communications system such as a mobile cellular communications system, a wireless local area network (WLAN), or fixed wireless access (FWA), communications nodes such as a base station (BS) or an access point (AP), a relay station (RS), and user equipment (UE) are generally capable of transmitting their own signals and receiving signals from other communications nodes. Because a radio signal is attenuated greatly in a radio channel, in comparison with a transmit signal of a local transmitting end, a signal coming from a communications peer becomes very weak when the signal arrives at a receive end. For example, a power difference between transmit power and receive power of a communications node in a mobile cellular communications system may be up to 80 dB to 140 dB or even greater. Therefore, to avoid self-interference caused by a transmit signal of a transceiver to a receive signal of the transceiver, radio signal transmission and reception are generally differentiated by using different frequency bands or different time periods. For example, in frequency division duplex (FDD) system, for transmission and reception, communication is performed by using different frequency bands separated by a guard band; in time division duplex (TDD) system, for transmission and reception, communication is performed by using different time periods separated by a certain guard period, where the guard band in the FDD system and the guard period in the TDD system are both used to ensure that reception and transmission are fully isolated and to avoid interference caused by transmission to reception.
Different from the conventional FDD or TDD technology, a wireless full duplex technology may implement operations of reception and transmission simultaneously on a same radio channel. In this way, spectral efficiency of the wireless full duplex technology is theoretically twice that of the FDD or TDD technology. Apparently, a precondition for implementing wireless full duplex lies in that strong interference (referred to as self-interference) caused by a transmit signal of a transceiver to a receive signal of the transceiver is avoided, reduced, or canceled as much as possible, so that no adverse impact is caused to proper reception of a wanted signal.
In a full duplex system, self-interference entering a receiver mainly includes two types of self-interference components.
A first-type self-interference component is a main-path self-interference component, and its power is relatively high. The main-path self-interference component mainly includes a self-interference signal that is leaked from a transmit end to a receive end due to leakage of a circulator, and a self-interference signal that enters the receive end due to antenna echo reflection. Conventional passive radio frequency self-interference cancellation is mainly used to cancel the first-type self-interference component. A path delay, power, and phase of this type of component depend on hardware itself such as an intermediate radio frequency unit and an antenna and a feeder of a specific transceiver. The path delay, power, and phase are basically fixed or change slowly, and it is unnecessary to perform fast tracing on each interference path of the first-type self-interference component.
A second-type self-interference component is mainly a self-interference component that is formed after a transmit signal is transmitted by a transmit antenna and encounters multi-path reflection at a scatterer or a reflection plane or the like in a spatial propagation process. When the full duplex technology is applied to scenarios such as a base station and a relay station in a cellular system, and a Wi-Fi access point (AP) disposed outdoors, because antennas of the devices are generally mounted relatively high, and there are few scatterers or reflection planes within a range of several meters to tens of meters around the devices, multi-path delays of multi-path reflected self-interference components that undergo spatial propagation, in the signals received by the devices, are relatively great and widely distributed, and with an increase in delays, power of corresponding multi-path signals (signals reflected from scatterers or reflection planes or the like that are far away) tends to decrease.
In the prior art, generally, an apparatus having a structure shown in FIG. 1 is used to cancel the second-type self-interference component in a manner of active analog self-interference cancellation or digital baseband self-interference cancellation. Specifically, a baseband digital self-interference signal reconstructed in a digital domain is reconverted to an analog domain by using a digital to analog converter (DAC), and then undergoes analog baseband processing (not shown in the figure) in the analog domain or is up-converted to an intermediate radio frequency, and is used to cancel a self-interference signal included in an analog receive signal; digital baseband self-interference cancellation in the digital domain is to use a reconstructed baseband digital self-interference signal to directly cancel a self-interference signal included in a digital receive signal in the digital domain. However, self-interference cancellation performance of the apparatus is finally limited by a dynamic range of an ADC (Analog-to-Digital Converter)/DAC (Digital-to-Analog Converter). Generally, the dynamic range of the ADC/DAC is about 60 dB. Therefore, when power of the second-type self-interference component is 60 dB higher than that of a wanted signal, the conventional method cannot be used to effectively cancel the second-type self-interference component.